[S] help with nested ANOVA

Bill Shipley (bshipley@courrier.usherb.ca)
Wed, 27 May 1998 10:53:15 -0400


I have been asked to analyse data from the following experiment. 50 radish
plants were independently grown in the presence of one of 10 genotypes of a
bacterium (5 replicates per genotype). The final weight of the radish plant
after 30 days was measured as the dependent variable. 5 of the bacterial
genotypes produces a toxin and the other 5 do not. The researcher wishes to
know if:
(1) those bacterial genotypes producing the toxin reduce the final size of
the radish relative to those that do not produce the toxin?
(2) whether there are differences between genotypes within a type
(presence/absence of toxin)?

It seems to me that this is a nested ANOVA with genotype nested within
"toxin type". There are (I think) three strata: toxin, genotype within
toxin, and replicates within genotype within toxin. If this is correct,
then I have two questions.

1. what is the proper error stratum for the "toxin type" term? Is it the
"genotypes within toxin" stratum?
2. what is the proper error stratum for the "genotypes within toxin" term?
Is it the "replicates within genotypes within toxin" term?
2. If so, is there a way in SPLUS to specify the proper error term for each
stratum?

Here is my code so far:
fit<-aov(weight~toxin+genotype/toxin)
Bill Shipley
Departement de Biologie
Universite de Sherbrooke
Sherbrooke (Quebec)
CANADA J1K 2R1
bshipley@courrier.USherb.ca
voix: 819-821-8000 poste 2079
telecopieur: 819-821-8049
Visitez notre site WEB : http://www.usherb.ca/SCES/BIO/index.html

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